1. Field of the Invention
The present invention relates to a particle size distribution measuring apparatus for measuring the distribution of particle size in a group of particles to be measured included in an object to be measured using an optical technique (for example, a laser diffraction/scattering means).
2. Description of Related Art
In laser diffraction/scattering-type particle size distribution measuring apparatuses, a group of particles to be measured (for example, a powder) in a dispersed state in a medium (for example, water or air) is irradiated with a parallel light beam so that the spatial distribution of the light intensity of the scattered light that has been diffracted/scattered by the group of particles to be measured can be detected by a number of photodetector elements and, thus, an arithmetic operation is carried out using this distribution of the light intensity on the basis of the Fraunhofer diffraction theory and the Mie scattering theory so as to calculate the distribution of particle size of the group of particles to be measured.
FIG. 5 is a schematic diagram showing an example of the structure of a conventional particle size distribution measuring apparatus. FIG. 5 is a combination of a schematic diagram showing the structure of an optical system and a block diagram showing the structure of a data sampling circuit and a signal processing system utilizing a computer. In addition, the direction of a line connecting a light source unit to a ring detector (photodetector) is the X direction, one direction perpendicular to the X direction is the Y direction, and the direction perpendicular to the X and Y directions is the Z direction.
A particle size distribution measuring apparatus 101 is provided with a cell base 31 on which a cell 30 containing an object to be measured S (mixture of a liquid medium L and a group of particles to be measured P) is placed, a light source unit 40 for irradiating the cell 30 with a parallel light beam, a condensing lens 51, a ring detector 52 for detecting the distribution of the light intensity, a data sampling circuit 60, and a computer (control unit) 170 for controlling the entire particle size distribution measuring apparatus 101.
The light source unit 40 is installed in the left side portion of the particle size distribution measuring apparatus 101 and, typically, a laser light source 41, a condensing lens 42, a space filter 43, and a collimator 44 are provided in this order starting from the left.
In this structure of the light source unit 40, a laser beam generated by the laser light source 41 passes through the condensing lens 42, the space filter 43 and the collimator 44 so as to become a parallel light beam, which is then directed in the frontal direction (X direction, from left to right in the figure) in order to irradiate the cell 30.
As a result, the parallel light beam is diffracted/scattered by the group of particles to be measured P within the cell 30 so as to provide a spatial distribution pattern of the light intensity of the diffracted/scattered light when an object to be measured S is contained inside of the cell 30 that has been placed on the cell base 31.
Meanwhile, a condensing lens 51 and a ring detector 52 are provided in the right side portion of the particle size distribution measuring apparatus 101 in this order starting from the left.
The ring detector 52 is made up of a number of (for example, 64) photodetector elements (photodiodes) having detecting surfaces in a ring or a semi-ring form of which the radii differ from each other, which are positioned in concentric circles with the optical axis of the condensing lens 51 at the center. Each photodetector element allows light to enter therein at a diffracted/scattered angle corresponding to the position thereof. Accordingly, the output signal of each photodetector element indicates the intensity of light at each diffracted/scattered angle.
In this structure of the condensing lens 51 and ring detector 52, the diffracted/scattered light is condensed on a detecting surface of the ring detector 52 through the condensing lens 51 so as to form a diffraction/scattering image in a ring form.
Thus, the output signal of the ring detector 52 is sequentially digitalized by the data sampling circuit 60 made up of an amplifier, a multiplexer and an A-D convertor so as to be transmitted to the multipurpose computer 170 as the data of the light intensity distribution of the diffracted/scattered light.
The computer 170 is provided with a CPU 180 and a memory 190, and a display unit 71 having a monitor screen and an input apparatus 72 having a keyboard 72a and a mouse 72b are linked to the computer 170. When the functions are divided into blocks based on the processing of the CPU 180, the CPU 180 has an object measuring unit 81 for measuring an object to be measured S and a calculation unit 83 for calculating the distribution of particle size of the group of particles to be measured P. In addition, the memory 190 has a light intensity distribution storage region 91 for storing the data of light intensity distribution and a basic data storage region 92 for storing the refractive indices of particles and water (liquid medium L), publicly known equations for arithmetic operations on the basis of the Fraunhofer diffraction theory and the Mie scattering theory, and the like.
When a user inputs an instruction to measure an object to be measured S to the input apparatus 72, the object measuring unit 81 controls the system in such a manner that the cell 30 that contains the object to be measured S is irradiated with a parallel light beam from the light source unit 40 and the light intensity data from the ring detector 52, that is to say, the data of light intensity distribution is acquired so as to be stored in the light intensity distribution storage region 91.
The calculation unit 83 uses the data of light intensity distribution that has been acquired by measuring the object to be measured S and the refractive indices of the particles and the liquid medium L so as to carry out a publicly known arithmetic operation on the basis of the Fraunhofer diffraction theory and the Mie scattering theory and, thus, controls the system so that the particle size distribution of the group of particles to be measured P included in the object to be measured S can be calculated.
In this particle size distribution measuring apparatus 101, the optical axis of the condensing lens 51 and the center axis (optical axis) of the ring detector 52 must be along the same line with a high precision. However, the optical axis of the condensing lens 51 and the center axis of the ring detector 52 may not align with each other and, therefore, the optical axis of the condensing lens 51 and the center axis of the ring detector 52 are adjusted relative to each other for each measurement.
In order to do so, a detecting surface for adjusting the optical axis is formed in the center portion of the ring detector 52. FIG. 6 is a diagram showing an example of the ring detector 52. In the ring detector 52 a number of (for example, 5) photodetector elements 52a having detecting surfaces in a semi-ring form with radii that differ from each other are placed in concentric circles, with a circular detecting surface 52b for adjusting the optical axis at the center. The detecting surface 52b for adjusting the optical axis is divided into two in the Y direction and, at the same time, divided into two in the Z direction so as to be made up of four photodetector elements 52b′. 
As a result, the ring detector 52 is shifted in the YZ directions by means of a drive mechanism (see FIG. 5) so that the intensity of the output signal that is outputted from each of the four photodetector elements 52b′ becomes equal and, thus, the optical axes are matched prior to measurement.
Another type of particle size distribution measuring apparatus has also been developed wherein an auto-alignment mechanism is provided with an actuator for shifting the collimator 44 in the direction perpendicular to the optical axis instead of shifting the ring detector 52 in the YZ directions and an optical axis adjustment processing unit for feeding a control signal for shifting the collimator 44 to the actuator on the basis of the output signal from the detecting surface for adjusting the optical axis (see Patent Document 1).